Valuable metals such as rare metals and noble metals are used in a wide range of industrial fields, and it is very important for Japan, where resources are poor, to stably secure valuable metals.
A major method for separation, recovery, and purification of valuable metals is solvent extraction. Solvent extraction uses an industrial extractant such as a phosphoric acid-based extractant, carboxylic acid-based extractant, or oxime-based extractant. Known representative examples of phosphoric acid based extractants include a di(2-ethylhexyl) phosphoric acid and its analog 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester; known representative examples of carboxylic acid-based extractants include neodecanoic acid; and known representative examples of oxime-based extractants include 2-hydroxy-5-nonylacetophenone oxime and 5,8-diethyl-7-hydroxy-6-dodecaoxime.
Further, the present inventors recently discovered that particular tetraalkylnitriloacetic acid diacetamide compounds and salts thereof are very preferable as extractants for extraction of metallic elements (see, for example, Patent Documents 1, 2, and 3). However, the synthesis methods for these extractants are complicated, and their production costs are high. Therefore, at present, the cost performances of these methods are rather poor.
Known examples of synthesis methods for tetraalkylnitriloacetic acid diacetamide compounds include a production method including the following Steps (a) to (c):
(a) a step of performing nucleophilic substitution reaction of a dialkylamine with a 2-halogenated acetyl halide to obtain a 2-halogeno-N,N-dialkylacetamide;

(b) a step of performing nucleophilic substitution reaction of iminodiacetic acid with the 2-halogeno-N,N-dialkylacetamide to obtain a nitrilotriacetic acid derivative;
and
(c) a step of performing amidation of one carboxyl group of the nitrilotriacetic acid derivative with a dialkylamine to obtain a tetraalkylnitriloacetic acid diacetamide compound;
wherein R1, R2, R3 and R4 independently represent the same or different hydrocarbon group, with the proviso that the total number of carbon atoms in the hydrocarbon groups R1, R2, R3, and R4 is 8 to 64.
Dichloromethane, which is used as a reaction solvent in this synthesis method, is designated as a harmful substance in the Act on the Evaluation of Chemical Substances and Regulation of Their Manufacture, etc.; Industrial Safety and Health Act; Air Pollution Control Act: Water Pollution Prevention Act; PRTR Law; and the like, and its use should be avoided. Further, chloroacetyl chloride is designated as a non-medical deleterious substance. Because of extremely high reactivity, it is highly irritative and corrosive. Chloroacetyl chloride drastically reacts with water to cause strong production of heat, and also production of hydrogen chloride. Therefore, its use should be avoided. Further, since a condensing agent water-soluble carbodiimide (WSC) is an expensive chemical, the production cost of tetraalkylnitriloacetic acid diacetamide synthesized using it as a raw material is high. This indicates that, while the efficiency of the extraction separation step using this compound can be increased due to excellent extraction separation performance, the metal extractant cost is high, so that the overall cost cannot be reduced.